1. Field of the Invention
The present invention relates to a photodetecting apparatus to be utilized in measurement of internal information relative to light in a scattering and absorption medium.
2. Related Background Art
Techniques of non-invasive measurement of an absorption coefficient or a transport scattering coefficient in a scattering and absorption medium by irradiating the scattering and absorption medium such as a living body with light such as pulsed light or continuous light and detecting light diffused during propagation in the scattering and absorption medium and thereafter emerging therefrom have been drawing attention. Especially, a method for obtaining internal information in the scattering and absorption medium by using pulsed light or intensity-modulated light as irradiation light, measuring a timing difference or a phase difference between the irradiation light and light emerging from the scattering and absorption medium after the irradiation light is diffused during propagation therein, and obtaining the internal information in the scattering and absorption medium from the information of phase difference is most effective among such techniques.
An apparatus utilizing this method is, for example, disclosed in "B. Chance: PHASE MODULATION SPECTROPHOTOMETRY, U.S. Pat. No. 4,972,331". This apparatus is irradiating a scattering and absorption medium such as a living body with modulated light resulting from modulating the intensity of light having a plurality of predetermined wavelengths by a predetermined modulation frequency, detecting light emerging therefrom with a photomultiplier tube (PMT) thereby to measure a phase difference between a phase of an ac component having a modulation frequency of modulated light upon irradiation and a phase of an ac component having a modulation frequency of detected light upon detection.
In the above-described photodetection, it is considered that an S/N ratio of the optically detected signal is proportional to the one half power of the number of detected photons. Accordingly, in the above-described apparatus, light having a wavelength at which the number of photons to be detected becomes the smallest, that is, light having a wavelength at which the intensity of output light becomes the smallest upon irradiation with the same light intensity, or modulated light with intensity at which the sufficient S/N ratio can be obtained for any kinds of scattering media needs to irradiate an object to be measured. With this radiation, a method for adjusting a gain of the photodetecting apparatus so that the optically detected signal is amplified to be readily processed later can be considered; however, in the photodetecting unit, a time between a light input and an output of an optically detected signal is different depending upon the gain. For example, the gain of the photomultiplier tube is adjusted by varying an applied voltage, but the variation of the applied voltage means the variation of the acceleration voltage, which changes the time between the light input and the output of the optically detected signal. Further, the gain of an avalanche photodiode (APD) is adjusted by varying a bias voltage, but the time between the light input and the output of the optically detected signal is varied because of variation of the bias voltage. FIG. 1 is a graph showing a relation between a bias voltage and a phase at a modulation frequency when one avalanche photodiode on the market is used and modulated light the intensity of which is modulated by a frequency of 40 MHz irradiates the avalanche photodiode. As shown in the graph, the phase difference is higher than 60.degree. at maximum depending on the bias voltage.
Therefore, for the stable measurement of the timing difference or phase difference, prior to the measurement, a function of gain of the photodetecting unit and time between the light input and the output of the optically detected signal needs to be precisely measured, and the timing difference or phase difference needs to be compensated for every gain upon the measurement.